Numerous so-called twin-wire formers of the above described type are known. For example, see German Offenlegungsschriften Nos. 2 534 290; 2 248 454 and 2 442 925. Machines of this construction have recently become increasingly adapted for the use at high speeds. The basic principle is to maintain the outer belt under a given tension during its travel, thereby to produce in the portion of the periphery of the forming cylinder which is wrapped, a pressure in the normal direction which removes water from the injected fiber suspension or the web of fibers being formed.
Paper webs produced on such twin-wire formers frequently exhibit many defects. Often, the edges of the webs have smaller thickness than their central regions, the fibers may be undesirably aligned in their longitudinal direction, and/or the web appears flaky, fluffy or cloudy when one looks through it. Ideally, the web should have approximately the same strength in both its longitudinal and its transverse directions, should not appear cloudy when looked through and should have a constant thickness profile measured over its width.